Improving brick-to-mortar bond strength by the addition of carbon fibers to the mortar

The addition of short carbon fibers in the optimum amount of 0.5% of the cement weight to mortar increased the brick-to-mortar bond strength by 150% under tension and 110% under shear when the gap between the adjoining bricks was fixed, and by 50% under tension and 44% under shear when this gap was allowed to freely decrease due to the weight of the brick above the joint. This effect is attributed to a decrease of the drying shrinkage by the fiber addition. The drying shrinkage decrease was particularly large at 2 to 24 h of curing. At 24 h, the shrinkage was decreased by 50% by the addition of fibers in the amount of 0.5% of the cement weight. Fibers in excess of the optimum amount gave less bond strengthening due to increased porosity in the mortar.